A number of different wireless communication techniques have been developed, including frequency division multiple access (FDMA), time division multiple access (TDMA) and various spread spectrum techniques. One common spread spectrum technique used in wireless communication is code division multiple access (CDMA) signal modulation in which multiple communications are simultaneously transmitted over a spread spectrum radio-frequency (RF) signal. Some example wireless communication devices that have incorporated one or more wireless communication techniques include cellular radiotelephones, PCMCIA cards incorporated within portable computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like.
Conserving power in a wireless communication device is a paramount concern. To conserve power, the wireless communication device may periodically operate in a low-power mode, often referred to as sleep mode. When operating in sleep mode, the wireless communication device can reduce power consumption by disconnecting power to selected internal components. For example, slotted paging techniques have been developed, in which paging signals are sent from a base station to wireless communication devices within assigned paging slots, separated by predetermined intervals of time. Slotted paging allows a wireless communication device to operate in sleep mode during the period of time between consecutive paging slots without missing paging signals.
A slot cycle refers to the amount of time between successive paging slots to be detected by a particular wireless communication device in a slotted paging system. Slot cycles are typically on the order of 1 to 20 seconds, but may be any length of time. For example, slot cycles for wireless communication devices operating in data transfer mode may be much longer than slot cycles associated with real time radiotelephone calls because the response time of the wireless communication device may not need to be as quick for wireless data transfer.
During the sleep period of the slot cycle, the wireless communication device tracks the amount of elapsed time in order to determine the appropriate time to wake up for reception of paging signals. Thus, even during sleep mode, the wireless communication device consumes some power, i.e., power sufficient to track the amount of elapsed time. The system clock of the wireless communication device is typically a relatively high frequency, high power clock such as a voltage controlled temperature compensated crystal oscillator (VCTCXO). Unfortunately, a VCTCXO or other high frequency, high quality clock may consume excessive amounts of power.
To further reduce power consumption during sleep mode, a wireless communication device may implement a relatively low frequency, low power clock that operates specifically during sleep mode, allowing the high power clock to be powered down along with other internal components of the wireless communication device during the sleep mode. However, low frequency, low power clock signals typically suffer from considerable variation and/or drift, making it more difficult to accurately track the amount of elapsed time during sleep mode.